A UWB (Ultra Wide Band) communication scheme, which is an impulse radio scheme, is one of high-speed radio transmission techniques. With the UWB communication scheme, pulse signal sequences formed with pulse signals synchronized with predetermined cycle timings are used for communication. Accordingly, the UWB communication scheme is employed in the ultra wideband communication.
For example, UWB communication is known to use pulse signals having a very narrow pulse width of about 1 nano second instead of using carrier waves. With this UWB communication, a base station (i.e. reader) transmits pulse signals having a very narrow pulse width, to communication terminals, so that it is possible to perform accurate positioning (hereinafter, also referred to as “UWB positioning”) of communication terminals in a multipath propagation environment.
However, with UWB communication, the upper limit of transmission power of a base station is determined from the perspective of interference against other systems. The power of signals transmitted from a base station attenuates due to propagation loss, and therefore a size of a communication area in a UWB communication system is practically limited. Further, communication terminals in a UWB communication system are generally small like tags. Therefore, in case where a battery is mounted on a communication terminal, it is assumed that the battery has a small current capacity like a button battery.
In view of above, in case where UWB positioning is performed in a desired area, communication terminals are naturally required to have adequate sensitivity and are furthermore required to employ configurations to realize power saving.
For example, Patent Literature 1 discloses a conventional positioning system. FIG. 1 shows a configuration of a tag disclosed in Patent Literature 1. The passive tag shown in FIG. 1 increases sensitivity with respect to an RF received signal based on a sensitivity control signal. Further, the tag disclosed in Patent Literature 1 adopts a scheme of back-scattering signals from a base station, that is, switches between the state where radio waves from a reader are not reflected by making load impedance of antennas matched (that is, the state where code “0” is transmitted) and the state where radio waves are reflected by making impedances unmatched (that is, the state where code “1” is transmitted). Note that the passive scheme is a scheme adopted by a tag without a power supply. Accordingly, a communication area of the passive scheme is very small. For example, a communication area by a UWB band may be 1 meter or less.
For example, Patent Literature 2 discloses a conventional UWB communication tag. FIG. 2 discloses a configuration of the UWB communication tag disclosed in Patent Literature 2. With the UWB tag shown in FIG. 2, the filtering section extracts UWB pulse components from a received signal, the detecting section detects envelopes or detects peaks, and the integrating section integrates the detection result over a predetermined integration period (for example, over a time slot). Then, the signal demodulating section demodulates the baseband signal by deciding the data value (between “1” and “0”) based on the decision criterion of whether or not the A/D conversion value of the integrated value acquired in the integrating section is equal to or greater than a threshold value. The UWB tag shown in FIG. 2 improves noise robustness by capturing synchronization while adjusting the integration period (i.e. time slot) in the integrating section.